This invention relates to an improved apparatus and technique for monitoring the oxygen concentration in a gas, e.g., in a combustible fuel environment.
Processes are known for testing the oxygen concentration of a sample gas. For example, U.S. Pat. No. 3,960,500 to Ross et al. discloses a zirconium oxide or zirconia analysis technique. To utilize this technique, it is necessary that the zirconia (ZrO.sub.2) in the form of a solid electrolyte be doped with a magnesia, yttria, calcia, or other bivalient molecule. The presence of these doping materials in the zirconium oxide solid creates vacancies of oxygen (valence -2) ions in a way similar to a doping process in a semi-conductor material. The doping effect occurs at temperatures ranging from 300.degree.-900.degree. C., but for most effective ion concentration, a temperature of approximately 815.degree. C. (1500.degree. F.) insure a stable and efficient crystal lattice structure.
The zirconia is coated on both sides with a platinum electrode which serves as an electrical contact for conducting electrical current between the solid electrolyte and an external circuit. On one electrode oxygen molecules of a reference gas are reduced by combining them with electrons to provide oxygen ions. On the other electrode oxygen ions from the zirconium yield up their electrons to form new oxygen atoms. The former reaction is known as the cathodic reaction and the latter known as the anodic reaction. These two reactions occurring at the cathode and anode of the detector create a current flow which is equivalent to a voltage difference across the element. By utilizing the well-known Nernst equation and observing the voltage difference created by the half cell reactions it is possible to determine the oxygen concentration of a sample gas.
The Nernst equation is of the form; EQU E=(RT/nF) 1n (P.sub.r /P.sub.s),
where R is equal to the universal gas constant, F is equal to Faradays' number, T is equal to absolute temperature, P.sub.r and P.sub.s are the oxygen partial pressures of the reference and sample gases respectively, and n is the number of electrons transferred in the half cell reaction. Since one knows the partial pressure due to the reference gas and also knows the other elements of the equation, it is possible by determining the voltage difference across the electrodes, to determine the partial pressures of the sample gas and thereby determine the oxygen concentration of the sample.
In the prior art (see the Ross patent) the temperature rise required for proper crystal lattice structure was provided by an external furnace. The external furnace resulted in a number of disadvantages including the following: (a) the furnace presented an additional cost to the total system apparatus cost; (b) the furnace was bulky and cumbersome to work with; (c) the oxygen analyzer was not maneuverable and could not be placed directly into the sample gas; (d) the time required to reach the operating temperature of the sample analyzer was long, (approximately 15 minutes); and, (e) the temperature distribution of the apparatus was uneven (it was hotter on the top than on the bottom).
The present invention eliminates the problems of the prior art by directly embedding within the crystal lattice structure of the zirconium oxide a resistance heater element to obtain the proper operating temperature. The heater element comprises a metallic grid structure upon which the zirconium oxide can be molded or coated in such a manner that the heater element in no way disrupts the crystal lattice structure of the sensor. The heater element is attached to a standard source of voltage and in no way adds to system design complexity since it is already necessary that electric connections enter the system to monitor the voltage differences and thereby observe concentration.
The technique of directly embedding the heater in the zirconium oxide results in a number of improvements over the prior technique. The cost of an embedded heater element and the process of sensor fabrication is much less than the cost of providing heat by furnace radiation.
The heater element is in direct contact with the sensor, consequently the size of the apparatus is greatly reduced thereby allowing the utilization of a compact insulator guard in the immediate region of the sensor to further insure ambient temperature is maintained at a relatively constant value.
Because the heater element meshes in intimate contact with the sensor crystal structure, the temperature rise can be attained much quicker and uniform temperature distribution is easier to achieve.
The input power necessary to provide the energy to heat the apparatus is reduced because of the direct physical contact between the heater and the sensor.
Finally, the heater element provides a rugged frame for sensor fabrication thereby improving the durability of this sensor.
The above and other features and advantages of the invention will become more apparent as the invention becomes better understood from the detailed description that follows, when considered in connection with the accompanying drawings.